The present invention relates to optical coherence tomography (OCT) systems and methods. In particular, the present invention relates to frequency domain optical coherence tomography (FD-OCT) systems and methods. In particular, the present invention relates to FD-OCT systems and methods capable of adjusting an axial field of view of structural data acquired from an object.
Optical coherence tomography (OCT) is an optical interferometric method for gaining structural information about an object. For this, the object is illuminated with measuring light having a particular spectrum indicating an intensity distribution of the measuring light in dependence of wavelengths comprised in the measuring light. Such a spectrum may be characterized by at least two parameters, a mean wavelength (or peak wavelength) and a spectral width of the spectrum. The mean wavelength may be obtained as a 0th moment of the spectrum. The spectral width of the spectrum may be considered as a spectral range around the mean wavelength, wherein an accumulated intensity of light having wavelengths within this wavelength range amounts to a major portion of a total intensity of the measuring light.
A coherence length of the measuring light is proportional to a ratio between the square of the mean wavelength and the spectral width. The coherence length of the measuring light determines under which conditions interference between two different light portions of the measuring light may be observed. Interference of two light portions of the measuring light can only be observed, if a difference of optical paths traveled by the two light portions differs by less than the coherence length. This property is exploited in different implementations of optical coherence tomography.
In different implementations of OCT the object is illuminated by at least a first portion of the measuring light having a low coherence length. Depending on the material comprised in the investigated object, the mean wavelength of the spectrum of the measuring light, and other physical properties, the intensity of the measuring light penetrating the object exponentially decreases characterized by a particular penetration depth. The penetration depth may amount for example to 1 mm to 3 mm for biological objects such as tissue, if the mean wavelength is in a range of for example 800 nm to 1300 nm. Measuring light penetrated into the object up to a particular depth within the object interacts with the material present within a volume of the object at that depth which comprises scattering and reflection processes. In particular, a reflectivity within a volume of the object depends on a refractive index and a gradient of the refractive index of the material within the particular volume of the object. Also, an orientation of an interface between two volume portions within the object having different refractive indices relative to a direction of a beam path of the measuring light influences the amount of reflected light emanating from the object in a particular direction. In OCT, typically measuring light returning from the object in a direction opposite to the direction of the incident measuring light is detected.
A second portion of the measuring light traverses an optical beam path which optical path length is controlled by a position of e.g. a reflecting surface. The first portion of the measuring light having interacted with the object at a certain depth and the second portion of the measuring light beam being reflected at the reflecting surface arranged at a certain position are superimposed and detected.
The different implementations of OCT differ in the way the object is probed at different depths and also in the way the detecting the superimposed measuring light is performed.
In time domain OCT (TD-OCT) probing different depths of the object (that is performing an axial scan) is performed by displacing the reflecting surface at which the second portion of the measuring light is reflected before superimposing the second portion with the first portion of the measuring light having interacted with the object. As explained above, only that part of the first portion of the measuring light that interacted with material in the object at a depth having traversed an optical path length whose difference to an optical path length traversed by the second portion of the measuring light is smaller than the coherence length of the measuring light will contribute to the detected intensity of the superimposed light. Thus, by displacing the reflecting surface structural data from different depths within the object may be acquired.
In frequency domain OCT (FD-OCT) the second portion of the measuring light is also reflected at a reflecting surface but this reflecting surface need not to be displaced in order to probe different depths of the object that means to perform an axial scan. Instead, structural information about the object in different depths, i.e. in particular reflectivities in different depths, are obtained by detecting intensities of the superimposed light in dependence of a wavelength of the superimposed light. For this, mainly two techniques or variations of OCT have been developed, namely spectral domain OCT (SD-OCT) and swept-source OCT (SS-OCT).
In spectral domain OCT the superimposed light comprised of a first portion of measuring light having interacted with the object and a second portion of the measuring light being reflected at a reflecting surface is spectrally dispersed using a spectrometer which e.g. detects the spectrally dispersed and thus spatially separated superimposed light by a positionally resolving detector. The positionally resolving detector typically comprises plural detector segments, wherein each segment receives a spectral portion of the superimposed light. Thereby, the positionally resolving detector supplies a spectrum of the superimposed light for further processing. By Fourier transformation of the spectrum of the superimposed light a distribution of reflectivities of the object along an axial direction, i.e. a depth direction, is obtained.
An axial field of view or depth field of view (FOV) of the SD-OCT method depends on a width of a spectral range of a spectral portion received by one of the segments of the positionally resolving detectors, i.e. the spectral resolution. The smaller the width of the spectral range of the spectral portion of the superimposed light received by a segment of the detector, the larger the axial field of view. In contrast thereto, an axial resolution or depth resolution of the obtained distribution of reflectivities within the object, i.e. the structural information, depends on a total spectral range of the superimposed measuring light detected by the plural segments of the positionally resolving detector.
In swept-source OCT (SS-OCT) a sweepable light source generating measuring light having a spectrum with a spectral width much narrower than a spectral width of a spectrum of measuring light used in spectral domain OCT is used to illuminate the object. A first portion of such measuring light returned from the sample and superimposed with a second portion of the measuring light is detected by a photo-detector without the need of previously spectrally dispersing the superimposed light. A spectrum of the superimposed measuring light at different wavelengths of the measuring light is obtained by sweeping a mean wavelength of the measuring light over time and concurrently detecting the superimposed light. An envelope of a spectrum of the measuring light cumulated over time during such sweeping may correspond to a typical spectrum of measuring light used in SD-OCT.
In SS-OCT the axial field of view (FOV) depends on a spectral width of the measuring light at every point in time, i.e. the maximum spectral resolution, e.g. achieved by time resolved detection.
From document US 2007/0024856 A1 an optical coherence imaging system is known, wherein an effective line width of the detected superimposed light can be reduced using periodic optical filters thereby increasing the axial field of view. However, accurate adjustments of the periodic optical filters are required.
Document DE 10 2005 046 690 A1 discloses a spectral domain OCT system, wherein a lens group having an adjustable focus length is arranged between a diffraction grating and a positionally resolving detector. Thereby, an axial field of view of the object can be varied by adjusting the focal length of the lens group and displacing the detector relative to the lens group.
However, the conventional OCT systems either require expensive components, elaborate adjustments of components, or exhibit disadvantages regarding sensitivity or a required total intensity of measuring light illuminating the object.
It is therefore an object of the present invention to improve conventional OCT systems and methods to enable effective measurement of sensitive objects.